1. Field of the Invention
This invention relates to the field of frequency down conversion, which is also referred to by such names as frequency translation, heterodyne action, mixing and beating. Specifically the invention relates to frequency conversion systems which have frequency errors introduced into the frequency converted signal because of environmental effects.
2. Description of the Prior Art
In a superheterodyne receiver, a received signal is converted to a lower frequency to facilitate the data demodulation and other signal measurement functions. This process of converting the signal to a lower frequency may incorporate multiple stages that convert the signal to successively lower frequencies, or in some cases a combination of conversions to higher and lower frequencies are used to ultimately bring the signal frequency down to the desired lower frequency. This process of frequency conversion can introduce significant phase and frequency errors into the signal.
Superheterodyne receivers have been used in space-based positioning systems where high frequency signals from satellites are detected, for example, on offshore drilling platforms, seismic boats and the like, where onboard computers provide immediate positioning data. Usually, one or more antennae are located on such platforms and boats in an open area so as to obtain, as much as possible, a clear view of the sky with an antenna. The signal detected at the antenna site on the platform or boat must be transmitted via a coaxial cable to a receiver so that it may be down converted in frequency, and ultimately demodulated.
Because the attenuation at the extremely high received carrier frequency, e.g., about 4 GHz, is excessive when such a high frequency signal is transmitted via coaxial cable, the received signal at the antenna site is down converted to a lower frequency so as to decrease signal attenuation for transmission via the cable to further down-conversion circuitry. Unfortunately, inexpensive local oscillators useful for modulating the received signal must be placed at a location which is subject to wide variations in temperature (e.g., from -20 to +70 degrees centigrade) and mechanical vibrations. Resonators useful as local oscillators for this application may drift up to plus or minus 3 MHz for such temperature variations. Mechanical vibration creates increased resonator phase noise and generates spurious output frequencies harmonically related to the frequencies of mechanical vibration. Offshore drilling platforms and seismic surveying boats are subject to considerable vibration from engine and ocean forces.